In recent years, miniaturized actuators have increasingly gained importance and have become indispensable in many fields of application. By using suitable designs as well as drive arrangements (electrostatic, electromagnetic, thermoelectric, piezoelectric) it is possible to implement actuators having very different properties which cover a broad range of applications.
However, there is a fundamental problem in creating large forces at large deflections—a property that is essential, in particular, in the field of fluid displacement and MEMS loudspeakers. This is due to the fact that actuators having large deflections may use low spring rigidities, whereas high spring rigidities tend to be used for transmitting large forces. An exception to this is constituted by electrodynamic drives only, which can create comparatively large forces and strokes even at low spring constants since the force is created by means of the magnetic field.
In particular in acoustics, the majority of all structural components therefore are based on electrodynamic drives. A classic example are precision-engineered loudspeakers for mobile phones wherein sufficiently large strokes and forces for air displacement are generated by means of a moving coil which moves within a permanent magnetic field.
Disadvantages of these conventional electrodynamic loudspeakers are the high power consumption of approx. 1 watt due to the low efficiency as well as large acoustic distortions. A further disadvantage is the relatively large structural height of 3-4 mm.
By means of MEMS technology, said disadvantages (low efficiency, large structural height) can be overcome. However, there are no MEMS loudspeakers on the market, but merely a series of publications by research laboratories. In the US 2013/0156253 A1 and in literature [Shahosseini et al., Optimization and Microfabrication of High Performance Silicon-Based MEMS Microspeaker, IEEE Sensors journal, 13 (2013) 273-284], an electrodynamic MEMS loudspeaker is described which, however, involves hybrid integration of a permanent magnetic ring. The concept of piezoelectric MEMS loudspeakers was presented in U.S. Pat. Nos. 7,003,125, 8,280,079, US 2013/0294636 A1 and in literature [Yi et al., Performance of packaged piezoelectric microspeakers depending on the material properties, Proc. MEMS 2009, 765-768] and [Dejaeger et al., Development and Characterization of a Piezoelectrically Actuated MEMS Digital Loudspeaker, Procedia Engineering 47 (2012) 184-187]. Therein, however, the piezoelectric materials such as PZT, AlN or ZnO have been directly applied onto the loudspeaker diaphragm, so that the properties of the drive and of the diaphragm are linked. A further piezoelectric MEMS loudspeaker comprising a plate-shaped body that is deflected out of the plane in the manner of a piston shape via a diaphragm and several actuators is presented in US 2011/0051985 A1. Digital MEMS loudspeakers based on arrays with electrostatically driven diaphragms, which however, can produce sufficiently high sound pressures only at high frequencies are described in U.S. Pat. No. 7,089,069, US 2010/0316242 A1 and in literature [Glacer et al., Reversible acoustical transducers in MEMS technology, Proc. DTIP 2013].